Wrinkled absorbent particles of high effective surface area having fast absorption rate

ABSTRACT

The subject invention pertains to large particle size, attrition-resistant, continuous, but wrinkled, high surface area aqueous fluid absorbent polymers, preferably of a crosslinked acrylic acid or acrylate. These absorbents have absorption rates superior to spherical absorbents. The subject invention further pertains to a process of preparing such polymers comprising suspension polymerization with an at least partially off phase soluble initiator. The subject invention further pertains to absorbent structures containing such polymers and to the use of such polymers therein.

The present invention relates to absorbents formed of water insoluble,typically gel-forming, polymeric materials that are capable of, forexample, absorbing many times their own weight upon contact with aqueousfluids. More particularly, the present invention focuses upon polymericmaterials that absorb substantial volumes of such fluids at a high rateof absorption, processes for making them, uses thereof in absorbentarticles, and absorbent articles incorporating such polymeric materials.

Many water insoluble gel-forming polymers are known for their usefulnessas absorbents because of their ability to imbibe and bind or immobilizeaqueous fluids. These polymeric materials find employment in industryfor various dewatering and fluid immobilization uses, such as waterretaining agents in agricultural/horticultural fields, dehydration ofoil, and like purposes. In recent years absorbent polymers having largecapacities for absorbing aqueous fluids have been developed and havefound use in personal care products for absorbing aqueous biologicalfluids. In a typical personal care product, such as a diaper, theaqueous fluid absorbent polymer is utilized in powder form, and is oftenmixed with cellulosic fibers that help initially absorb and distributethe fluid load. The polymeric materials of interest in such products arebased upon a variety of polymers including those derived from watersoluble ethylenically unsaturated monomers or graff polymers in whichunsaturated monomers are graff polymerized onto a polysaccharide (suchas starch or cellulose) or other polymeric backbone.

A preferred absorbent material is derived from a water insoluble gelformed by copolymerizing an ethylenically unsaturated carboxylic acidwith a multifunctional crosslinking monomer. The acid monomer or polymeris substantially neutralized with an alkali metal hydroxide, dried andpulverized into a powder form before use in a personal care product. Apreferred polymer gel is a copolymer of acrylic acid/sodium acrylate andany of a variety of crosslinkers.

Achieving desired polymeric characteristics that provide superiorperformance in a personal care product has long been a challenging goalof researchers. The product must perform for the user but also must becapable of being economically and safely made. At the customer/userlevel, a diaper, for example, most desirably must keep the usersubstantially dry, even in response to repeated wettings. Thus, a keydesired characteristic of an aqueous fluid absorbent polymer, at leastfor diaper use, is that it have high fluid capacity, e.g., a centrifugecapacity greater than 10 grams/gram. In addition, a most desiredcharacteristic of the aqueous fluid absorbent polymer is that it has afast rate of absorption, adequate to imbibe and hold the fluid duringabsorption without leakage of fluid from the device in which it isemployed. It is adequate fast rate, while maintaining all of the otherdesired qualities of the aqueous absorbent, that has eluded priorresearchers.

It is well-known in the prior art that the rate of fluid absorbency issubstantially determined by the surface area of the particles. Thus,extremely fine particles of aqueous fluid absorbent, those less than 100mesh (149 micrometers), would be expected to absorb liquids at a rapidrate. However, the individual particles at the surface of the polymericmass of such fines initially contacted by the fluid rapidly swell andadhere together, drastically reducing the rate of absorption as thefluid is "gel blocked" from access to particles of absorbent in the massthat are more remote from the surface. Lumps or "fish eyes" often formsuch that overall performance of the absorbent material isunsatisfactory.

An additional difficulty with fine materials is that such fines createdusting problems in manufacturing and forming into finished articles.Fines or dusts may be a source of industrial hygiene risks for workersas well as pose difficult materials handling problems. At the productlevel it may be difficult to immobilize fine particulate materials inthe article or device of interest without elaborate containmentstructures that increase costs and may limit the ability of the articleto perform at a desired level.

A number of workers have attempted to produce a non-dusting and/or fastabsorbency rate product by making somewhat larger particles that stillpossess useful absorbency, changing polymer particle surfacecharacteristics or adhering fines particles together. The balancing ofdesirable product end-use qualities with manufacturing limitations hasheretofore meant accepting one or more less desirable characteristics.

Yamasaki, et al. in U.S. Pat. No. 4,446,261 describes making a largersized particle that avoids dusting and is said to have improved capacityand absorbency rate over the prior art. The process produces resinparticles by means of a suspension polymerization process, including awater-soluble redox initiator system, that utilizes oil solublecellulose ester or cellulose ether as a protecting colloid in order toobtain spherical particles of a size that do not cause dusting. However,while stating that the beads have a fast absorbency rate over the priorknown suspension processes, only absorbency rates greater than 4 minutesfor 0.5 grams of polymer to absorb 5 milliliters of saline solution, arereported.

A number of researchers have worked at improving absorption rates forwater absorbent resins. Nakamura, et al. in U.S. Pat. No. 4,683,274suggests rate improvements for α,β-unsaturated carboxylic acid basedpolymers produced by inverse emulsion polymerization through inclusionof a sucrose fatty acid ester as a protective colloid agent. Waterabsorption rates are said to improve to about 1 minute from 15 minutesfor the time required for 1 gram of resin to absorb 30 milliliters of0.9 percent aqueous sodium chloride solution. Such improvements, whilesubstantial, are still not sufficient for many personal care productuses.

In GB 2119384A, crosslinking the surface layer of a specific waterabsorbing resin having a carboxyl group with a polyhydric alcohol issaid to minimize fines and gel blocking. However, while the improvementreported is significant, much higher absorption rates are still desired.

In agglomeration of fines techniques, maintaining adequate absorbencygenerally requires that the degree of adherence or binding of the finesparticles together be limited, similar to the well-known limits oncrosslinking. Otherwise, fused particles are formed that may result inproducts that still gel-block. Limiting binding of the particlestogether generally produces agglomerated products that tend to be easilyattrited during incorporation into finished articles, recreating thedusting problems. Attrition may also occur in the finished product suchthat fines block distribution of fluid in an article by filling inchannels and spaces in a fiber matrix.

It appears clear from the difficulties of making an agglomerated productthat a desirable absorbent polymer particulate, especially for use inpersonal care products applications, remains a discreet large-sizedparticle, if, in contrast to the prior art, adequate absorbent capacityand rate could be achieved. While large particle sized absorbents havebeen made, as noted above, they simply heretofore have not possessedadequate rates of absorbency. Thus, it remains desirable to provide anabsorbent polymer that is a discreet non-agglomerated particulate thathas high absorbent capacities, does not gel block and has a fast rate ofabsorbency, i.e., a vortex rate of 1 minute or less.

The present invention is a fluid absorbent polymer having a fast rate ofabsorption for aqueous fluids, comprising individual polymer particleseach having a high surface area, each said particle surfacesubstantially continuous but including a plurality of wrinklescomprising folds, ridges, crevices, and channels. The polymer particlesare swellable such that a fluid in contact with said particles iseffectively exposed to a substantial portion of the surface area of saidparticles for absorption of said fluid. The wrinkles of the invention,including folds, ridges, crevices, and channels, are furthercharacterized and depicted in FIGS. 1 through 4. The resulting producthas a rate of absorption of less than one minute, preferably less than20 seconds, as measured by a "vortex test" described below. The particlesize of the fluid absorbent polymer particles is substantially greaterthan 75 micrometers in diameter, preferably greater than 100micrometers.

The invention also includes a method for making the fast rate aqueousfluid absorbent of the invention, comprising:

suspending a water soluble monomer mixture in a continuous, inert,organic liquid phase, containing suspending agents, with agitation suchthat droplets of said monomer mixture form in said continuous phase;

adding to said suspension an oxidizing component of a redox initiatorpair, said reducing component at least partially soluble in the organicliquid phase; and

introducing a reducing component of said redox pair into said organicphase under polymerizing conditions at a controlled rate such that saidpolymer particles having wrinkled surfaces form.

The invention also includes the use of the fluid absorbent polymer ofthe invention in an article for absorbing biological fluids. Theinvention includes such use wherein the article is a disposable diaper.

The invention also includes an absorbent article comprising ahydrophilic fiber material and any of the polymers of the invention. Theabsorbent article preferably further comprises a fluid impervious bottomlayer and a fluid pervious top layer, said absorbent article beingretained between said bottom layer and said top layer; said absorbentarticle being further characterized as containing between 20 and 98weight percent of said hydrophilic fiber material and between 2 and 80weight percent of any of said polymers of the invention.

The absorbent polymer of the invention are particularly useful informing personal care articles for absorbing biological fluids, such asdisposable diapers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a photomicrograph at a magnification of 50 of the polymerparticles of the invention wherein the initial monomer mixture comprises24 percent acrylic acid.

FIG. 1(b) is a photomicrograph of the particles of 1(a) in a crosssection at a magnification of 50.

FIG. 2(a) is a photomicrograph at 50 magnification of the polymerparticles of the invention wherein the initial monomer mixture comprises26 percent acrylic acid.

FIG. 2(b) is a photomicrograph at 50 magnification showing the particlesof FIG. 2(a) in cross section.

FIG. 3(a) is a photomicrograph at 50 magnification showing the polymerparticles of the invention wherein the initial monomer concentration ofthe polymerization mixture is 29 percent acrylic acid.

FIG. 3(b) is a photomicrograph at a magnification of 50 showing theparticles of FIG. 3(a) in cross section.

FIG. 4(a) is a photomicrograph at a magnification of 50 showing thepolymer particles of the invention wherein the initial monomerconcentration is 33 percent acrylic acid.

FIG. 4(b) is a photomicrograph at a magnification of 50 showing thepolymer particles of FIG. 4(a) in cross section.

FIG. 5 is a photomicrograph at a magnification of 50 showing the polymerparticles of Comparative Example A, representative of the prior art.

A key element of the invention is a high surface area aqueous absorbentpolymer particle that is characterized by a continuous but wrinkledsurface and an exceptionally high rate of absorbency for aqueous fluids,in comparison with conventional spherical absorbent materials. Referringto the Figures, FIGS. 1-4 are photomicrographs of the polymer particlesofthe invention, at a magnification of 50, for a crosslinked polymerbased upon acrylic acid, as further described in the examples below.Each polymer particle is also shown in cross section which furthersuggests itsuniqueness and exceptionally high surface area of the resinavailable for absorbing aqueous fluids. Herein, the phrase "particlesurface substantially continuous but including a plurality of wrinklescomprising folds, ridges, crevices, and channels" is defined to mean thesurface morphology depicted in FIGS. 1-4.

The polymer particles of the invention have a high surface area.Preferred areas are between 0.3 and 0.5 m² /g. An especially preferredsurface area is 0.3 m² /g.

The high surface area polymer of the invention is made by means of asuspension polymerization process of the invention. A key feature of theprocess of the invention for making the high surface area, highabsorbencyrate polymer, is that polymerization of the discontinuousphase monomer droplets suspended in a continuous oil phase is initiatedin the continuous oil phase by means of an initiator that is at leastpartially oil phase soluble. This is in contrast to conventionalsuspension polymerizations which generally employ initiators soluble inthe aqueous phase.

Generally, the monomer droplets must be of a certain minimum size foradequate wrinkling to occur. It is preferred that the monomer dropletsbe larger than 75 micrometers in diameter at initiation of thepolymerization. More preferably, the monomer droplets will be between 75and 1000 micrometers in diameter, most preferably between 150 and 1000micrometers, and most preferably with an average diameter of 400micrometers at the initiation of polymerization.

The water absorbent compositions of the invention may be made from avariety of polymers or copolymers. Basically, any water-solubleethylenically unsaturated monomer or mixture thereof that crosslinks toform a substantially water insoluble gel or particle is suitable.Crosslinked structures may be obtained by the copolymerization of awater-soluble monomer and a crosslinking monomer possessing at least twopolymerizable double bonds in a molecular unit, as is well-known in theart. Monomer mixtures that include graft, as well as additionpolymerizingsystems may be employed so long as the wrinkled surface canbe imparted to the resulting polymer or copolymer particles.

Exemplary water-soluble monomers include ethylenically unsaturatedamides such as acrylamide, methacrylamide, and fumaramide as well astheir N-substituted derivatives. Ethylenically unsaturated carboxylicacids suchas acrylic, methacrylic, and crotonic acids and their saltsare preferred. Suitable polycarboxylic acids include maleic acid andfumaric acids and itaconic acid. Preferred ethylenically unsaturatedcarboxylic acid esters include hydroxyethylacrylate,hydroxyethylmethacrylate, and esters of acrylic and methacrylic acidswith polyethylene oxide. Vinyl amines such as vinyl pyridine and vinylmorpholine, and diallyl amines are also useful. Other suitable monomersare well known to those skilled in the artas discussed in U.S. Pat. No.4,708,997.

The ethylenically unsaturated monomer may be partially neutralized asset forth below. In such cases, the monomer mixture will furthercomprise the salt of the ethylenically unsaturated monomer. The monomermixture may also include components that graft polymerize onto one ormore other monomer additional monomers of the monomer mixture.Polysaccharides, such as starch and cellulose are examples ofgraft-polymerizable components. Particularly suitable is agraft-polymerizable polyvinyl alcohol.

The concentration of monomer in the monomer mixture used to make thewrinkled particles of the invention is a key factor in determining thedegree of wrinkling achieved. The effect of concentration of monomer, ofcourse, depends upon the particular system of interest. In the preferredacrylic acid/sodium acrylate system, varying the concentration ofacrylic acid monomer in the initial monomer mixture, has a profoundeffect upon the degree of wrinkling, as demonstrated by FIGS. 1-4,wherein the concentration of monomer varies between 24 and 33 weightpercent. For the acrylic acid/sodium acrylate system at least about 10percent acrylic acidin the initial monomer mixture (prior toneutralization) is required to achieve useful wrinkling, useful at leastfrom an economic view. Preferably, the initial monomer mixture willcontain from 10 to 50 percentacrylic acid, more preferably from 24 to 35percent acrylic acid.

The suspension polymerization of the present invention, when carboxylicacid monomers are employed, generally provides that the monomers beneutralized at least partially prior to the polymerization. Preferably,the acid monomers will be between 75 and 95 percent neutralized, morepreferably between 80 and 90 percent neutralized. The neutralization isgenerally carried out, as is well known in the art, by simply mixing themonomers, including any crosslinking agents, with any suitable base,e.g. an alkali hydroxide such as sodium hydroxide or potassium hydroxideor an alkali carbonate or bicarbonate such as sodium or potassiumcarbonate or bicarbonate, as the initial step of the process ofpreparation of the polymers of the invention. The neutralization isadvantageously carried out at temperatures below about 40° C.,preferably below about 35° C.

The monomer mixture typically includes one or more crosslinking monomerswhich comprise organic compounds having two or more ethylenic groupscopolymerizable with the water-soluble monomers of the monomer mixture.Exemplary crosslinking monomers include diacrylate or dimethacrylate ofethylene glycol, diethylene glycol, triethylene glycol, tetraethyleneglycol, propylene glycol, 1,4-butane diol, 1,5-pentane diol, 1,6-hexanediol, neopentyl glycol, trimethylol propane and pentaerythritol;triacrylates or trimethacrylates of trimethylol propane andpentaerythritol tetracrylates or tetramethacrylates of pentaerythritol,N,N'-methylene-bis-acrylamide; N,N'-methylene-bis-methacrylamide; andtriallyl isocyanurate. Preferred crosslinkers includemethylene-bis-acrylamide, trimethylol propanetriacrylate and diethyleneglycol diacrylate and tetraethylene glycol diacrylate. Crosslinkingmonomers are present in the dispersion of water-soluble monomer in anamount effective to crosslink the water-soluble polymer. Typically, thecrosslinking monomer is used in amounts ranging from about 0.0001 toabout5 parts by weight, based on 100 parts by weight of thewater-soluble monomer used.

In the suspension polymerization process of the invention, the monomermixture is suspended in an inert organic phase or oil phase comprisingan organic material that is non-reactive with the monomers and resultingproducts. The water-immiscible oil phase of the suspension generallycomprises as least one inert hydrophobic liquid, such as a liquidhydrocarbon or substituted liquid hydrocarbon. Preferred organic liquidsare the halogenated hydrocarbons such as perchloroethylene, methylenechloride, and liquid hydrocarbons having 4 to 15 carbon atoms permolecule, including aromatic and aliphatic hydrocarbons and mixturesthereof such as benzene, xylene, toluene, mineral oils, liquid paraffinssuch as kerosene, and naphtha. Of the foregoing organic liquids, thehydrocarbons are the more preferred, with the aliphatic hydrocarbonsbeingmost preferred. A preferred commercially available aliphatichydrocarbon isISOPAR® M deodorized kerosene, sold by Exxon.

The inert organic or oil phase includes dispersing agents to keep theaqueous soluble monomer droplets suspended in an oil phase for thesuspension polymerization. These dispersing agents include surfaceactive materials such as sucrose fatty acid esters and/or polyglycerolfatty acidesters. Also included are nonionic surface active agentshaving HLB values of from 2 to 6. Polymeric materials useful asdispersants include the various cellulose ethers, such as ethylcellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropylcellulose, carboxyethyl cellulose and combinations thereof. Preferably,such cellulose ethers will be provided at a concentration of from 0.1 to2 weight percent, based on the weight of the monomer, more preferably0.5 weight percent based on the weight of the monomer. Other usefulmaterials include the hydrophobic clays such as cationic surfactanttreated bentonite clays. The preferred dispersing agent is a mixture ofa fumed hydrophobic silica (such as AEROSIL™ R-972 fumed silicamanufactured by Degussa, Inc.) and a copolymer of lauryl methacrylateand acrylic acid. In a preferred copolymer, the mole ratio of laurylmethacrylate to acrylic acid in the copolymer is 99 to 1.

As indicated above, a key element of the invention is the wrinkling andtheother surface and internal characteristics of the polymer particlesas shown in FIGS. 1-4. Such wrinkling is attributable to crosslinking ofthe monomer droplet in the vicinity of its surface. Introducing thewrinkling and high surface characteristics to the particles of theinvention is thought to be related to at least the process of theinvention which requires the utilization of an initiator system that isat least partiallyoil phase soluble. Preferably, a redox system isemployed which comprises, as a reducing component thereof, an at leastpartially oil phase soluble material. "Partially soluble in the organicliquid phase" is defined to mean possessing sufficient solubility in theoil phase to yield polymer particles as depicted in any of FIGS. 1-4.Preferably, the reducing agent will partition such as to provide between10 and 2500 ppm reducing agent in the oil phase, more preferable atleast 100 ppm reducing agent in the oil phase. Suitable oxidizingcomponents include, for example, t-butyl hydroperoxide (t-BHP);2,5-dihydroperoxy-2,5-dimethylhexane; and cumene hydroperoxide. Apreferred reducing component of the redox system is sulfur dioxide gas.Thermal initiators that have sufficient oil phase solubility, such asVAZO™ 64 azobisisobutyronitrile available from DuPont and benzoylperoxide are also suitable.

In the process of the invention, the monomer mixture is suspended in theinert oil phase, typically utilizing agitation, to form monomer mixturedroplets. The monomer droplets must be large enough such that thewrinkling characteristic imparted to the particles is useful, forexample,in increasing absorbency rate when employed in a personal careproduct. As is well-known in the art, variation of agitation intensityand shear may be used to control monomer droplet size. Generallypreferred is a droplet size of on the order of greater than 75micrometers. Maintaining proper droplet size requires limiting agitatorspeed, generally to less than thatutilized in the prior art, dependingupon blade design and other equipment limitations.

The reaction is carried out at any convenient temperature at which theinitiator system operates efficiently. Thus, in the preferred t-butylhydroperoxide/SO₂ system, the temperature of reaction may range fromless than room temperature to 90° C. Preferably the reaction isinitiated at room temperature or lower, preferably about 20° C.;proceeds adiabatically (to a temperature between 55° C. to 65° C.); andoptionally includes a final temperature increase to 75° C. The reactionis initiated by bubbling the reducing agent, such as sulfur dioxide,into the reaction mixture.

The rate of initiation should be such as to lead to an efficientpolymerization without compromising polymer properties. Excessively fastrates of initiation lead to shorter polymer chain lengths and inferiorpolymer properties. The rate of initiation may be adjusted by increasingor decreasing the flow rate of the reducing agent (to increase ordecreasethe rate of initiation, respectively). Preferred flow rates area function of the reactor system employed and may be determined byconventional means. For the 2-liter reaction systems described in theexamples and for reducing agents comprising a 0.1 percent mixture ofsulfur dioxide in nitrogen, suitable reducing agent flow rates arebetween 200 and 1000 mL/min. An especially preferred reducing agent flowrate under such conditions is 750 mL/min.

After the polymerization reaction is finished, the polymer product isrecovered by removing the inert oil phase and drying. The dried,finished product may then be treated with a wetting agent, such asVORANOL® 2070 polyol, manufactured by The Dow Chemical Company. Thewetting agent helps overcome the adverse effect of any remainingdispersing agent, such as hydrophobic inert inorganic silica material,remaining on the finished product. Preferably, the dry polymer beadswill be provided with 0.25 to 2weight percent wetting agent based on theweight of the dry beads, more preferably 0.5 to 1.5 weight percent. Morepreferably, 1 weight percent wetting agent based on the weight of drypolymer will be provided.

In characterizing the finished water-absorbent polymers of theinvention, swelling capacity under pressure (SCUP), centrifuge capacity,vortex rate,swell time, and surface area are measured. Preferredpolymers will have a low vortex rate and swell time and a high SCUP andswelling capacity.

Swelling capacity under pressure is closely related to the modulus of apolymer. SCUP measures the ability of a polymer to swell against apressure of 0.29 psi (2 kPa) which pressure simulates the pressures thatthe polymer must swell against in a disposable infant diaper. In thetest procedure, 0.160 gram of the polymer of interest is placed in acylinder including a Whatman GF-A filter paper resting on a 100 mesh(149 micrometers) screen. A loose fitting cover is placed on top of thepolymerand a 100 gram weight is placed on top of the cover. The polymeris exposedto 0.9 percent NaCl solution by means of an apparatus thatmaintains the level of the saline reservoir such that there is nohydrostatic head pressure. The amount of liquid absorbed with 30seconds, 5 minutes, and 60minutes is measured. Preferred 5 minute SCUPvalues are greater than 10 grams saline solution/gram polymer, are morepreferably greater than 13 grams saline solution/gram polymer, are evenmore preferably greater than 15 grams, and are most preferably greaterthan 20 grams saline solution/gram polymer.

The centrifuge capacity of the product of the invention is determinedusingthe following procedure. A pulverized sample of dry polymer isscreened to pass through a 30-mesh (500 micrometers) screen but to beretained on a 50-mesh (300 micrometers) screen. A 0.2 gram portion ofthis material is evenly placed in a 64 mm by 57 mm bag of a non-wovenfabric shaped like a tea bag. The bag containing the test material isimmersed in a 0.9 percentsodium chloride saline solution for thirtyminutes, removed from the solution, centrifuged at a speed of 1500 rpmfor 3 minutes, removed from the centrifuge and weighed. Centrifugecapacity is calculated as (W₃ -B₁)-(W₂ -W₁)!/(W₂ -W₁), where W₁ is theweight of an empty dry tea bag, W₂ is the weight of an empty dry tea bagcontaining the sample, W₃ is the weight of the wet centrifuged sampleand tea bag, and B₁ is the average weight of a wet centrifugedtea bag.Preferred centrifuge capacities are greater than 10 grams salinesolution/gram polymer, are more preferably greater than 15 grams salinesolution/gram polymer, are even more preferably greater than 20 gramssaline solution/gram polymer, and are most preferably greater than 25grams saline solution/gram polymer.

The rate at which the polymers of interest will absorb aqueous fluid isa key advantage of the invention. It is characterized by a "vortex rate"test. This test measures and reports in seconds the time required for avortex generated by a magnetic stir in a container to disappear when 2grams of absorbent polymer are added to 50 milliliters of 0.9 percentsaline solution. Preferred vortex rates are less than 65 seconds, aremorepreferably less than 40 seconds, and are even more preferably lessthan 20 seconds.

The swell rate of the polymer is determined using the followingprocedure. A pulverized sample of the dry polymer is screened to passthrough a 30-mesh (500 micrometers) screen, but to be retained on a50-mesh (300 micrometers) screen. A 1.0 gram portion of this material isplaced into a weighing boat. To the boat, 30.0 grams of a 0.9 percentsodium chloride saline solution is rapidly added. The swell time equalsthe time it takes for the sample to swell to form a continuous barrieratop the surface of the saline solution. Preferred swell times are lessthan one minute, are more preferably less than 35 seconds, and are evenmore preferably less than 25 seconds, and are most preferably less than10 seconds.

The polymers of the invention may be utilized as a principal absorbingcomponent of a personal care product. Atypical such product is adisposable diaper wherein the polymer of the invention is contained in acomposite structure generally comprising an exterior impervious sheetand a porous interior sheet, with the polymer of the invention,typically mixed with cellulose fibers, sandwiched between said sheets.

Other absorbent structures into which the polymers of the invention maybe utilized include incontinence devices, sanitary napkins, papertowels, andfacial tissues.

The following examples illustrate the products and process of theinvention. It is not intended that the invention be limited to theirscope.

EXAMPLE 1 Preparation of Wrinkled Particles of the Invention (29 PercentAcrylic Acidin the Aqueous Phase, Neutralized to 85.3 Percent Na Salt)

To 116 grams of acrylic acid is added 100 grams of a 50 percent aqueoussolution of sodium hydroxide, 0.23 grams of methylene-bis-acrylamide (anaqueous phase soluble crosslinking agent), 0.46 grams (4000 ppm) oftrimethylolpropane triacrylate (an oil phase soluble crosslinkingagent), 183 grams of water, and 0.46 grams of VERSENEX 80® chelatingagent, ofThe Dow Chemical Company. The monomer mix is cooled to 25° C.and added to a mixture of 0.6 grams of AEROSIL™ R-972 fumed hydrophobicsilica of Degussa, Inc., 0.32 grams of a copolymer of laurylmethacrylateand acrylic acid in a weight ratio of 99:1 as a dispersing agent, and800 grams of ISOPAR® M deodorized kerosene of Exxon in a 2 literreactor. The reactor is equipped with a 4-bladed agitator rotating at250 rpm. Then0.089 grams of 70 percent t-butyl hydroperoxide (t-BHP) isadded. The suspension is purged for 30 minutes with nitrogen and thenheated to 50° C. At 45° C., the polymerization is initiated by bubblinginto the suspension of 0-1 weight percent a dilute stream of sulfurdioxide in nitrogen at a flow rate between 200 mL/min and 1000 mL/min.The reaction temperature adiabatically rises to 55° C. After thepolymerization is complete, the reaction mixture is heated at 75° C. forone hour. The ISOPAR® deodorized kerosene is removedby filtration andthe polymer product of the invention dried in an oven. When dry, asoptional steps, the polymer is slurried in methanol and 0.58 grams ofVORANOL® 2070 polyol wetting agent of The Dow Chemical Company is added.The methanol is removed by vacuum stripping at 50° C. The wrinkledpolymer particles of the invention are characterized in Table II andFIG. 3.

EXAMPLES 2-4

Polymerizations identical to Example 1 are run at other acrylic acidmonomer concentrations. The recipes for those examples appear in TableI. These wrinkled polymer particles of the invention are characterizedin Table II and FIGS. 1, 2 and 4.

Referring to FIGS. 1-4, the wrinkled polymer particles, as a function ofpercent acrylic acid in the monomer mixtures, are shown. The pairedphotomicrographs at 50 magnification show the wrinkled polymer particlesof the invention for 24 percent initial acrylic acid monomer mixtureconcentration in FIG. 1(a). The same polymer particles are shown in FIG.1(b) wherein the particles have been immobilized and cross-sectioned.Considering the Figures together, it is seen that the degree ofwrinkling and internal complexity of the structure of the polymerparticles of the invention increase with increasing concentration ofmonomer.

                                      TABLE I                                     __________________________________________________________________________                               VERSENEX 80 ®                                                                      METHYLENE BIS-                                                                         TRIMETHYLOL PROPANE              EXAMPLE                                                                             % ACRYLIC                                                                            ACRYLIC ACID                                                                          50% NaOH                                                                            CHELATING                                                                              ACRYLAMIDE                                                                             TRIACETIC ACID                   NO.   ACID   (g)     (g)   AGENT (g)                                                                              (g)      (g)                              __________________________________________________________________________    1     29     116     110   0.46     0.23     0.46                             2     24      96      91   0.38     0.19     0.38                             3     26     104      98   0.41     0.21     0.41                             4     33     132     125   0.52     0.26     0.52                             __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    WRINKLED POLYMER PARTICLES OF THE INVENTION USING t-BUTYL HYDROPEROXIDE       AS INITIATOR                                                                  EXAMPLE                                                                             % ACRYLIC                                                                            30 SEC. SCUP                                                                          5 MIN. SCUP                                                                          60 MIN. SCUP                                                                         CENT. CAP.                                                                          VORTEX                                                                              SWELL TIME                                                                           SURFACE AREA            NO.   ACID   (g/g)   (g/g)  (g/g)  (g/g) (sec) (sec)  (m.sup.2 /g)            __________________________________________________________________________    1     29     8.5     20.6   22.5   20.5  9.4   9      0.32                    2     24     7.4     22.9   25.8   28.5  18.2  21.3   0.45                    3     26     8.3     25.3   26.3   25.4  10.3  11.7   0.26                    4     33     4.1     14.4   17.2   15.5  18.9  20.1   0.32                    __________________________________________________________________________

Comparative Example A

The polymerization of Example 1 is repeated with regard to process andreaction components, except the initiator system is of the prior art. Inthis experiment, the t-BHP oil phase soluble oxidizing component of theredox initiator system of the invention is replaced with an aqueoussoluble oxidizing component. Thus, 0.86 gram of sodium persulfate isaddedto the aqueous reaction mixture before the mixture is added to theISOPAR® M deodorized kerosene oil phase. The persulfate initiates thepolymerization at 45° C. and the reaction is allowed to proceedadiabatically at 55° C. for 2 hours, followed by an additional one hourheating at 75° C.

The product produced by the process of Comparative Example A issubstantially spherical in shape and round in cross section, as shown inFIG. 5. The vortex rate for the spherical beads is 145 seconds.

Comparing the vortex rate results of the Examples 1 through 4 productsof the invention reported at Table II with that of Comparative Example Ashows that the present invention compositions achieve significantlysuperior absorbency rates of 9 to 21 seconds. Thus, the products of theinvention are 7 to 16 times faster than Comparative Example A,characteristic of the prior art.

EXAMPLES 5 AND 6

The process of Example 1 is repeated except that2,5-dihydroperoxy-2,5dimethylhexane is substituted for t-BHP as theoxidizing component of the redox initiator system. The wrinkled polymerparticles of the invention are characterized in Table III.

                                      TABLE III                                   __________________________________________________________________________    WRINKLED POLYMER PARTICLES OF THE INVENTION USING 2,5-DIHYDROPEROXY-2,5-      DIMETHYLHEXANE AS INITIATOR                                                   SAMPLE                                                                             30 SEC. SCUP                                                                         5 MIN. SCUP                                                                          60 MIN. SCUP                                                                         CENT. CAP.                                                                          VORTEX RATE                                                                           SWELL TIME                            NO.  (g/g)  (g/g)  (g/g)  (g/g) (sec)   (sec)                                 __________________________________________________________________________    5    7.4    15.5   15.9   18.7  22.0    25.3                                  6    3.8    13.4   14.4   17.7  20.0    21.7                                  __________________________________________________________________________

EXAMPLES 7-9

The process of Example 1 is repeated except that cumene hydroperoxide issubstituted for t-BHP as the oxidizing component of the redox initiatorsystem. The wrinkled polymer particles of the invention arecharacterized in Table IV.

                                      TABLE IV                                    __________________________________________________________________________    WRINKLED POLYMER BEADS OF THE INVENTION USING                                 CUMENE HYDROPEROXIDE AS INITIATOR                                             SAMPLE                                                                             30 SEC. SCUP                                                                         5 MIN. SCUP                                                                          60 MIN. SCUP                                                                         CENT. CAP.                                                                          VORTEX RATE                                                                           SWELL TIME                            NO.  (g/g)  (g/g)  (g/g)  (g/g) (sec)   (sec)                                 __________________________________________________________________________    7    8.1    25.3   26.8   25.3  32.0    33.5                                  8    5.5    25.2   27.8   27.0  41.0    34.5                                  9    3.2    19.9   26.1   28.4  64.0    62.9                                  __________________________________________________________________________

EXAMPLE 10 Preparation of Polymer Particles of the Invention IncludingGraftable Monomers in the Monomer Mixture (29 Percent Acrylic Acid, inthe Aqueous Phase, Neutralized to 85.3 Percent Na Salt)

To 116 grams of acrylic acid are added 110 grams of sodium hydroxide,0.93 gram of diethylene glycol diacrylate, 0.46 (4000 ppm) gram oftrimethylolpropane triacrylate (TMPTA), 183 grams of water containing5.0 grams of AIRVOL™ 107 (polyvinyl alcohol), and 0.46 gram of VERSENEX®80. The monomer mix is cooled to 25° C. and added to a mixture of 0.6gram of AEROSIL™ R-972 fumed hydrophobic silica, 0.32 gram of acopolymer of laurylmethacrylate and acrylic acid and 800 grams ofISOPAR® M deodorized kerosene in a 2 liter reactor. The reactor isequipped with a 4-bladed agitator rotating at 250 rpm. Then 0.089 gramof 70 percent t-butyl hydroperoxide is added. The suspension is purgedfor 30minutes with nitrogen and then heated to 50° C. At 25° C., thepolymerization is initiated by bubbling in a dilute steam of sulfurdioxide in nitrogen. The reaction temperature proceeds adiabatically at55° C. After the polymerization is complete, the reaction mixture isheated at 75° C. for one hour. The ISOPAR® deodorized kerosene isremoved by filtration and the polymer product dried in an oven. Whendry, the polymer is, as an optional treatment, slurried in methanol and0.58 gram of VORANOL® 2070 polyol wetting agent is added.The methanol isremoved by vacuum stripping at 50° C.

The product is characterized by a vortex rate of 14 seconds.

EXAMPLE 11 Preparation of Wrinkled Particles of the Invention (31.6Percent Acrylic Acid in the Aqueous Phase, Neutralized to 80 PercentSodium Salt)

To 126.4 grams of acrylic acid is added 112.26 grams of a 50 percentaqueous solution of sodium hydroxide, 1.74 grams (4000 ppm) oftrimethylolpropane triacrylate (an oil phase soluble crosslinkingagent), 155.19 grams of water, and 0.46 grams of VERSENEX 80® chelatingagent,of The Dow Chemical Company. The monomer mix is cooled to 25° C.andadded to a mixture of 0.6 grams of AEROSIL™ R-972 fumed hydrophobicsilica of Degussa, Inc., 0.32 grams of a copolymer of laurylmethacrylateand acrylic acid in a weight ratio of 99:1 as a dispersing agent, and800 grams of ISOPAR® M deodorized kerosene of Exxon in a 2 literreactor. The reactor is equipped with a 4-bladed agitator rotating at250 rpm. Then, 0.9 grams of 7 percent t-butyl hydroperoxide (t-BHP) isadded. The suspension is purged for 30 minutes with nitrogen and thenheated to 50° C. At 45° C., the polymerization is initiated by bubblinginto the suspension a dilute stream of 0.1 weight percent sulfur dioxidein nitrogen at a flow rate of 750 mL/min. The reactiontemperatureadiabatically rises to 55° C. After the polymerization iscomplete, the reaction mixture is heated at 75° C. for one hour. TheISOPAR® deodorized kerosene is removed by filtration and the polymerproduct of the invention is dried in an oven. When dry, as optionalsteps,the polymer is slurried in methanol and 1 weight percent ofVORANOL® 2070 polyol wetting agent of The Dow Chemical Company based onthe weight of the dry polymer is added. The methanol is removed byvacuum stripping at 50° C. The polymer produced exhibits a 30 secondSCUP of 5.2, a 5 minute SCUP of 7.5, a 60 minute SCUP of 23.4, acentrifuge capacity of 32.3 grams/gram and a vortex rate of 13.5seconds.

EXAMPLE 12 Preparation of Wrinkled Particles of the Invention (35Percent Acrylic Acidin the Aqueous Phase, Neutralized to 80 PercentSodium Salt)

The procedure of Example 10 is repeated except that 140.0 grams acrylicacid, 124.34 grams of the 50 percent aqueous solution of sodiumhydroxide,128.85 grams water, 0.193 grams trimethylolpropanetriacrylate, and 0.5 grams of the 7 percent t-butyl hydroperoxide(t-BHP) are utilized rather than the amounts set forth in Example 10.The polymer produced exhibits a 30 second SCUP of 11.6, a 5 minute SCUPof 24.7, a 60 minute SCUP of 26.3,a centrifuge capacity of 24.6grams/gram and a vortex rate of 18.0 seconds.

EXAMPLE 13 Preparation of Polymer Particles of the Invention IncludingGraftable Monomers in the Monomer Mixture (27 Percent Acrylic Acid, inthe Aqueous Phase, Neutralized to 75 Percent Na Salt)

To 108 grams of acrylic acid are added 89.93 grams of a 50 percentaqueous solution of sodium hydroxide, 0.54 gram of tetraethyleneglycoldiacrylate,4.64 (43000 ppm) gram of trimethylolpropane triacrylate(TMPTA), 184 grams of water containing 32.1 grams of a 15 percentsolution of AIRVOL™ 107 polyvinyl alcohol of Air Products, and 0.46 gramof VERSENEX® 80 chelating agent. The monomer mix is cooled to 25° C. andadded to amixture of 0.6 gram of AEROSIL™ R-972 fumed hydrophobic silicaof Degussa, Inc., 0.32 gram of a copolymer of lauryl methacrylate andacrylicacid and 800 grams of ISOPAR® M deodorized kerosene in a 2 literreactor. The reactor is equipped with a 4-bladed agitator rotating at250 rpm. Then 0.39 gram of 7 percent t-butyl hydroperoxide is added. Thesuspension is purged for 30 minutes with nitrogen and then heated to 50°C. At 25° C., the polymerization is initiated by bubbling in a dilutestream of sulfur dioxide in nitrogen at a rate of 750mL/min. Thereaction temperature proceeds adiabatically to 55° C. After thepolymerization is complete, the reaction mixture is heated at 75° C. forone hour. The ISOPAR® M deodorized kerosene is removed by filtration andthe polymer product dried in an oven. When dry, the polymer is, as anoptional treatment, slurried in methanol and 1 weight percent VORANOL®2070 polyol wetting agent based on the weight of the dry polymer isadded. The methanol is removed by vacuum stripping at 50° C.

The product is characterized by a 30 second SCUP of 3.6, a 5 minute SCUPof10, a 60 minute SCUP of 27, a centrifuge capacity of 31 grams/gram anda vortex rate of 29 seconds.

EXAMPLE 14 Preparation of Polymer Particles of the Invention IncludingGraftable Monomers in the Monomer Mixture (35 Percent Acrylic Acid, inthe Aqueous Phase, Neutralized to 75 Percent Na Salt)

The procedure off Example 12 is repeated except that 140.0 grams acrylicacid, 116.57 grams of the 50 percent aqueous solution of sodiumhydroxide,95.01 grams water, 0.58 grams trimethylolpropane triacrylate,41.61 grams of a 15 percent solution of AIRVOL™ 107 polyvinyl alcoholand 0.25 grams of the 7 percent t-butyl hydroperoxide (t-BHP) areutilized rather than the amounts set forth in Example 12, Further, inthis example no second crosslinking agent, i.e., tetraethyleneglycoldiacrylate, is used. The polymer produced exhibits a 30 second SCUP of2.1, a 5 minute SCUP of 22, a 60 minute SCUP of 24, a centrifugecapacity of 22 grams/gram and a vortex rate of 24 seconds.

What is claimed is:
 1. A fluid absorbent polymer having a fast rate ofabsorption, comprising:individual non-agglomerated polymer particles,each said particle having a mean surface area greater than about 0.2 m²/g, each said particle having a surface substantially continuous butincluding a plurality of wrinkles comprising folds, ridges, crevices andchannels.
 2. The polymer of claim 1, wherein said individualnon-agglomerated polymer particles have a mean surface area betweenabout 0.2 and about 0.5 m² /g.
 3. The polymer of claim 3, wherein saidindividual non-agglomerated polymer particles have a mean surface areaof about 0.3 m² /g.
 4. The polymer of either of claims 1 or 2, whereinsaid polymer is the polymerization product of a water-solubleethylenically unsaturated monomer mixture or salt thereof.
 5. Thepolymer of claim 4, wherein said ethylenically unsaturated monomermixture or salt thereof comprises an amide, carboxylic acid or itsesters, vinyl amine, or salt or mixture thereof.
 6. The polymer of claim5, wherein said ethylenically unsaturated monomer mixture or saltthereof comprises acrylic acid, sodium acrylate or mixture thereof, saidpolymer being crosslinked with a polyvinyl monomer.
 7. The polymer ofclaim 5, wherein said monomer mixture includes a monomer capable ofgraft polymerizing with at least one other component of said monomermixture.
 8. The polymer of claim 7, wherein said monomer capable ofgraft polymerizing with at least one other component of said monomermixture is polyvinyl alcohol.
 9. The polymer of either of claims 1 or 2,wherein said individual non-agglomerated polymer particles have a meandiameter greater than about 75 microns.
 10. The polymer of claim 9,wherein said polymer has a vortex rate of absorption less than about 60seconds.
 11. The polymer of either of claims 1 or 2, wherein saidindividual non-agglomerated polymer particles have a mean diameterbetween about 150 and about 1000 microns.
 12. The polymer of either ofclaims 1 or 2, wherein said individual non-agglomerated polymerparticles have a mean diameter of about 400 microns.
 13. The polymer ofeither of claims 1 or 2, further comprising a wetting agent.
 14. Thepolymer of claim 13, wherein said wetting agent is a polyol.
 15. Aprocess for making a fluid absorbent polymer having a fast rate ofabsorption, comprising:suspending an aqueous mixture of a water solublemonomer in a continuous, inert organic liquid phase containing adispersing agent with agitation such that droplets of said aqueousmixture form in said continuous phase; adding to said suspension anoxidizing component of a redox initiator pair, said oxidizing componentat least partially soluble in the organic liquid phase; and introducinga reducing component of said redox pair into said organic phase underpolymerizing conditions at a controlled rate such that said polymerparticles having wrinkled surfaces form.
 16. The process of claim 15,wherein said droplets have a mean diameter greater than about 75microns.
 17. The process of claim 15, wherein said droplets have a meandiameter between about 150 and about 1000 microns.
 18. The process ofclaim 15, wherein the droplets have a mean diameter of about 400microns.
 19. The process of any of claims 16, 17, or 18, wherein saiddispersing agent is a cellulose ether, said cellulose ether beingprovided in an amount of about 0.5 weight percent based on the weight ofthe water soluble monomer.
 20. The process of any of claims 16, 17, or18, wherein said dispersing agent is a nonionic surface activecomposition having a HLB value of from about 2 to about
 6. 21. Theprocess of any of claims 16, 17, or 18, wherein said dispersing agent ina mixture of hydrophobic silica and a copolymer of lauryl methacrylateand acrylic acid in a weight ratio of about 99 parts by weight laurylmethacrylate to about 1 part by weight acrylic acid.
 22. The process ofclaim 21, further comprising treating said polymer particles with awetting agent.
 23. The process of any of claims 16, 17, or 18, whereinsaid water soluble monomer comprises an ethylenically unsaturatedmonomer and crosslinker therefor, said ethylenically unsaturated monomerbeing at least partially neutralized by a basic material.
 24. Theprocess of claim 23, wherein said ethylenically unsaturated monomer isan amide carboxylic acid or its esters, vinyl amine, or salt or mixturethereof.
 25. The process of claim 23, wherein said ethylenicallyunsaturated monomer is acrylic acid.
 26. The process of claim 23,wherein said ethylenically unsaturated monomer is neutralized to adegree of about 75 to about 95 percent by said basic material.
 27. Theprocess of claim 23, wherein said ethylenically unsaturated monomer isneutralized to a degree of about 80 to about 90 percent by said basicmaterial.
 28. The process of claim 23, wherein said crosslinker ismetheylene-bisacrylamide, diethylene glycol diacrylate,tetraethyleneglycol diacrylate, trimetholpropane triacrylate, ormixtures thereof.
 29. The process of claim 23, wherein said oxidizingcomponent is t-butyl hydroperoxide, cumene hydroperoxide, or2,5-dihydroperoxy-2,5-dimethylhexane.
 30. The process of claim 23,wherein said reducing component is sulfur dioxide.
 31. A polymerproduced in accordance with the process of claim
 23. 32. The polymer ofclaim 31, wherein said ethylenically unsaturated monomer is acrylicacid.
 33. A method for using the polymer of claim 1 comprising retainingsaid polymer between a fluid impervious bottom layer and a fluidpervious top layer.
 34. The method of claim 33, wherein said individualnon-agglomerated polymer particles have a mean surface area betweenabout 0.2 and about 0.5 m² /g.
 35. A method for using the polymerproduced in accordance with the process of claim 16 comprising retainingsaid polymer between a fluid impervious bottom layer and a fluidpervious top layer.
 36. The method of claim 35, wherein said dropletshave a mean diameter greater than about 75 microns.
 37. The method ofclaim 35, wherein said droplets have a mean diameter between about 150and about 1000 microns.
 38. The method of claim 35, wherein saiddroplets have a mean diameter of about 400 microns.
 39. A polymer havinga fast rate of aqueous fluid absorption comprising individualnon-agglomerated polymer particles, each said particle having a meansurface area of between about 0.2 and 0.5 m² /g, a mean diameter greaterthan about 75 microns, and a surface which is substantially continuousbut including a plurality of wrinkles, comprising folds, ridges,crevices and channels, the polymer comprising polymerized units of oneor more water-soluble ethylenically unsaturated monomers or saltsthereof, and one or more crosslinking monomers.